John Sottery, president of IMS, Inc., and a leading sunscreen researcher, offers the following explanation:
Natural sunlight contains, among other things, ultraviolet (UV) photons. These photons are shorter in wavelength and higher in energy than visible light. Because they fall outside the visible spectrum, the human eye cannot perceive them. When it comes to sun exposure, however, what you can't see will hurt you. When these high-energy photons strike your skin, they generate free radicals and can also directly damage your DNA. Over the short term, this UV-induced damage can produce a painful burn; over the long term it causes premature aging of the skin, as well as millions of new cases of skin cancer each year.
The UV rays that we are exposed to here on the earth's surface consist of UVB and UVA photons. The shorter wavelength UVB rays don't penetrate deeply into skin; they cause significant damage to DNA and are the primary cause of sunburn and skin cancer. The longer wavelength UVA rays penetrate the deeper layers of skin, where they produce free radicals. UVA exposure has been linked to premature aging of the skin and immunologic problems.
A sunscreen product acts like a very thin bulletproof vest, stopping the UV photons before they can reach the skin and inflict damage. It contains organic sunscreen molecules that absorb UV and inorganic pigments that absorb, scatter and reflect UV. To deliver a high level of protection, a sunscreen product must have sufficient quantities of these protective agents and it must optimally deploy them over the skin's peaks and valleys.
The term SPF that appears on sunscreen labels stands for Sun Protection Factor, but it is really a sunburn protection factor. Products with a higher SPF allow fewer of the photons that produce sunburn to strike the skin. In simple terms, you can view an SPF 10 sunscreen as allowing 10 out of every 100 photons to reach the skin and an SPF 20 product as allowing only 5 out of every 100 photons to reach the skin. Because sunburn is primarily a UVB effect, it is possible for a sunscreen product to deliver high SPF while allowing a significant percentage of the incident UVA photons to reach the skin. To deliver true broad spectrum protection, products must also block a significant fraction of the UVA photons. In the U.S. market, this requires that the products contain significant levels of zinc oxide, avobenzone or titanium dioxide.